Rosa Zambonini was elected to represent the Wishaw ward on North Lanarkshire Council, replacing SNP colleague Marion Fellows.
In Falkirk, Paul Garner held a seat in Denny and Banknock ward for the SNP.
The Labour candidate in the by-election was suspended pending an investigation over sectarian comments he was said to have made on social media.
Andy Bell's name remained on the ballot paper for the election of the new Denny and Banknock councillor.
However, a Labour spokesman said the party had "withdrawn its support".
The by-election was called after John McNally was elected MP for Falkirk at May's general election.
In Falkirk, first-preference voting was: SNP 2,576, Lab 549, C 431, Green 170. The swing from Labour to the SNP was 23%.
In North Lanarkshire, first-count voting in the Wishaw ward was: SNP 1,915, Lab 1,230, C 385, Soc 117, Ukip 67, LD 37. Swing from Lab to SNP 21%.
The SNP has won by-elections in Falkirk and North Lanarkshire sparked by former councillors becoming MPs.