In May 2014 he won a special election to serve as an at-large member of Philadelphia City Council replacing outgoing councilmember Bill Green.

[5] Neilson was considered for the 5th Senatorial District seat after John Sabatina resigned on December 31, 2021, after being elected to the Philadelphia Court of Common Pleas the previous month.

[6] After a meeting of Democratic leaders asked to not be considered and Ward Leaders Shawn Dillon, Connie Dougherty, Alan Butkovitz, Pat Parkinson, Jim Donnelly, Bobby Henon, John Del Ricci, Harry Enggasser, Pete McDermott and John Sabatina Sr decided to support Shawn Dillion.

O'Brien, who had comfortably held the 169th District seat in the State House for several decades, resigned from the General Assembly the following January.

In the special election, he faced Dave Kralle, a long-time aide to O'Brien who served as his Chief of Staff at the end of his legislative tenure.

[14] Neilson entered the race to succeed Bill Green, and won the special election to defeat Matt Wolfe, a Republican ward leader from West Philadelphia.