There were 2 by elections held in 1961. The first by election was held on 29 April 1961 and the nomination day was held on 11 March 1961 while the second by election was held on 15 July 1961 with the nomination day held on 10 June 1961.
Former PAP minister Ong Eng Guan resigned his seat in Hong Lim, filing the famous "16 resolutions" in the legislative assembly against the government and challenged the PAP to defeat him there after his sacking from the cabinet. Shortly after, he was expelled by the party after making open disputes with his Cabinet colleagues, including over the abolishment of the City Council when he was the last Mayor. Two other PAP members had followed him to join his faction and resigned from the party but did not resign their seats with Ong.
It was held to elect a new assembly member after the death of the incumbent PAP member Baharuddin Mohammed Ariff in the constituency of Anson.
The victory of former Labour Front chief minister David Marshall, now Workers' Party (WP) leader, symbolised WP's first presence in the legislature.
At the same time, Ong Eng Guan mustered the two former PAP assembly members who resigned with him to form the United People's Party while independent Assembly Speaker A. P. Rajah joined the newly formed Singapore Alliance - an alliance of SPA, UMNO, MCA and MIC. Hence, for the first time since, there were no independent legislators.
Ong's landslide victory was attributed to his popularity with Hong Lim voters and his oratory skills.
The PAP candidate, Jek Yeun Thong was Ong's secretary during his time as Mayor in the City Council. Jek would be elected to parliament in the 1963 election and serve as a cabinet minister until 1984.
As both Ismail from Liberal Socialist Party and Ibrahim from Singapore Congress have failed to secure the minimum 12.5% of the votes, both had their deposits forfeited.