Defender Kilgallon made 27 appearances for Rovers this season, though 21 of those came in 2015, while Williamson pulled on the shirt 34 times.
That was enough to earn both of them an extra year with Gary Bowyer’s men.
However, veterans David Dunn and Paul Robinson will leave the club.
Dunn announced his 27-year association with Rovers would be ending in April while goalkeeper Robinson exits Ewood Park after eight years.
The former England international only made seven appearances this season and did not feature after September.
Chris Taylor and Luke Varney, who faces six months out with a ruptured Achilles, have also been told they are free to find new clubs.
Under-21s captain David Carson earns a one-year deal but Bradley Bauress, Kellen Daly, Adrian Edo, Sam Jones, Paulo Jorge, George Pierce, Cameron Scott, Yann Songo’o and Callum Williams all leave the club.