Company: NMI
Filing Date: 2025-07-07
Form Type: N-CSRS
Source: 0001193125-25-155990
Chunk: 0

Company: NUVEEN MUNICIPAL INCOME FUND INC
Filing Date: 2025-07-07
Form: N-CSRS
Chunk 0
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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM N-CSR CERTIFIED SHAREHOLDER REPORT OF REGISTERED MANAGEMENT INVESTMENT COMPANIES

| Investment Company Act file number 
 811-05488                          |

Nuveen Municipal Income Fund, Inc. (Exact name of registrant as specified in charter) Nuveen Investments 333 West Wacker Drive Chicago, Illinois 60606 (Address of principal executive offices) (Zip code) Mark L. Winget Vice President and Secretary 333 West Wacker Drive Chicago, Illinois 60606 (Name and address of agent for service) Registrant’s telephone number, including area code: ( 800) 257-8787 Date of fiscal year end: October 31 Date of reporting period: April 30, 2025

| Item 1. | Reports to Stockholders. |

Nuveen Municipal Closed-EndFunds This semi-annual report contains the Funds’ unaudited financial statements.

| Nuveen Municipal Value Fund, Inc.  |     | NUV |
| Nuveen AMT-Free Municipal Value    
 Fund                               |     | NUW |
| Nuveen Municipal Income Fund, Inc. |     | NMI |

Semi-annual Report

Table of Contents

| Important Notices                                             |     |  3 |
| Common Share Information                                      |     |  4 |
| About the Funds’ Benchmarks                                   |     |  6 |
| Fund Performance and Holdings                                 |     |  7 |
| Portfolios of Investments                                     |     | 14 |
| Statement of Assets and Liabilities                           |     | 56 |
| Statement of Operations                                       |     | 57 |
| Statement of Changes in Net Assets                            |     | 58 |
| Financial Highlights                                          |     | 60 |
| Notes to Financial Statements                                 |     | 64 |
| Additional Fund Information                                   |     | 74 |
| Glossary of Terms Used in this Report                         |     | 75 |
| Statement Regarding Basis for Approval of Investment Advisory 
 Contract                                                      |     | 76 |

2

Important Notices

Portfolio manager commentaries:The Funds include portfolio manager commentary in their annual shareholder reports. For your Fund’s most recent annual
portfolio manager discussion, please refer to the Portfolio Managers’ Comments section of the Fund’s annual shareholder report.

Fund changes:For
changes that occurred to your Fund both during and after this reporting period, please refer to the Notes