**Exhibit 31.2**

Certification Pursuant to Section 302 of the Sarbanes-Oxley Act of 2002

I, Michael P. McGrath, Vice President, Treasurer and Chief Financial Officer of the registrant, certify that: 

| 1.     | I have reviewed this   quarterly report on Form 10-Q of Mercantile Bancorp, Inc.;                                                                                                                                                                                                                                         |
|--------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
|   |                                                                                                                                                                                                                                                                                                                      |   |
| 2.     | Based on my knowledge,   this report does not contain any untrue statement of a material fact or omit   to state a material fact necessary to make the statements made, in light of   the circumstances under which such statements were made, not misleading with   respect to the period covered by this report;        |
|   |                                                                                                                                                                                                                                                                                                                      |   |
| 3.     | Based on my knowledge, the   financial statements, and other financial information included in this   report, fairly present in all material respects the financial condition,   results of operations and cash flows of the registrant as of, and for, the   periods presented in this report;                           |
|   |                                                                                                                                                                                                                                                                                                                      |   |
| 4.     | The registrant’s other   certifying officers and I are responsible for establishing and maintaining   disclosure controls and procedures (as defined in Exchange Act Rules   13a-15(e) and 15d-15(e)) for the registrant and have:                                                                                   |
|   |                                                                                                                                                                                                                                                                                                                      |   |
|   | a)                                                                                                                                                                                                                                                                                                                        | designed such disclosure   controls and procedures, or caused such disclosure controls and procedures to   be designed under our supervision, to ensure that material information   relating to the registrant, including its consolidated subsidiaries, is made   known to us by others within those entities, particularly during the period   in which this report is being prepared; |
|   |                                                                                                                                                                                                                                                                                                                      |   |
|   | b)                                                                                                                                                                                                                                                                                                                        | evaluated the   effectiveness of the registrant’s disclosure controls and procedures and   presented in this report our conclusions about the effectiveness of the   disclosure controls and procedures, as of the end of the period covered by   this report based on such evaluation; and |
|   |                                                                                                                                                                                                                                                                                                                      |   |
|   | c)                                                                                                                                                                                                                                                                                                                        | disclosed in this report   any change in the registrant’s internal control over financial reporting that   occurred during the registrant’s most recent fiscal quarter (the registrant’s   fourth fiscal quarter in the case of an annual report) that has materially   affected, or is reasonably likely to materially affect, the registrant’s   internal control over financial reporting; and |
|   |                                                                                                                                                                                                                                                                                                                      |   |
| 5.     | The registrant’s other   certifying officer(s) and I have disclosed, based on our most recent   evaluation of internal control over financial reporting, to the registrant’s   auditors and the audit committee of the registrant’s board of directors (or   persons performing the equivalent functions): |
|   |                                                                                                                                                                                                                                                                                                                      |   |
|   | a)                                                                                                                                                                                                                                                                                                                        | all significant   deficiencies and material weaknesses in the design or operation of internal   control over financial reporting which are reasonably likely to adversely   affect the registrant’s ability to record, process, summarize and report   financial information; and |
|   |                                                                                                                                                                                                                                                                                                                      |   |
|   | b)                                                                                                                                                                                                                                                                                                                        | any fraud, whether or not   material, that involves management or other employees who have a significant   role in the registrant’s internal control over financial reporting. |

Date: November 14, 2005

| /s/ Michael P. McGrath                                |   |
|-------------------------------------------------------|--------|
|                                                       |   |
| Michael P. McGrath                                    |   |
| Vice President, Treasurer and Chief Financial Officer |   |