Sydenham by-election
Results of the by-election that was held in Sydenham ward on Thursday 6 May 2021.
| Candidate | Party | Votes | Status |
| CROFT Helena | The Conservative Party Candidate | 982 | Not elected |
| LAVERY Jack | Labour and Co-operative Party | 2634 | Elected |
| LEE Nick | The Green Party | 820 | Not elected |
| WAYNE Michael Anthony | Lewisham People Before Profit | 188 | Not elected |
| WILSON Margot | Liberal Democrats | 513 | Not elected |
| Reason for rejected ballot paper | Number of ballot papers rejected |
| Want of an official mark | 0 |
| Voting for more candidates than the voter was entitled to | 7 |
| Writing or mark by which the voter could be identified | 0 |
| Unmarked or void for uncertainty | 73 |
| Total | 80 |
| Total eligible electorate | Ballot papers received (including rejected votes) | Turnout (%) |
| 11, 576 | 5,217 | 45.07 |