**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.;                                                                                                                                                                                                                                         |
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| 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;        |
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| 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;                           |
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| 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:                                                                                   |
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|   | 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; |
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|   | 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 |
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|   | 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 |
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| 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): |
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|   | 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 |
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|   | 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. |