Originally drafted by Brisbane with pick 8 in the National Draft, St Kilda moved to the Saints at the end of the 2013 season.
The 25-year-old has played 65 games for the Saints, but only five in 2018.
“I’m really looking forward to another year with the Saints,” Longer told the St Kilda website.
“It’s already been a great off-season for me, as I’ve been able to address a few lingering injuries. Now I feel better than I have in a long time and I’m just excited to get on the track with the boys and get to work.”
Pierce re-signs with the Saints after playing four games in 2018.
The 23-year-old said he hopes to repay the faith the Saints have shown in him.
“I was given the opportunity to prove myself at AFL level those last few weeks of the season and I am really keen to repay the faith the club has shown in me.” Pierce said.
“Billy and I have actually been doing some running sessions together over the past few weeks, so hopefully this drives us to both become better players.
“I’m confident that with another pre-season behind me I’ll be able to put my best foot forward and really push my case for more senior football.”