Seater was born and raised in Toronto, Ontario. He attended the Etobicoke School of the Arts and worked as a child actor.
Seater began acting in the late 1990s and appeared in the short films Night of the Living and The Stone Skipper, and had minor roles in the direct-to-video film Future Fear and the made-for-television films Rescuers: Stories of Courage: Two Couples, Vanished Without a Trace, On Thin Ice, Dirty Pictures, Cyber Seduction: His Secret Life, the Hanukkah-themed Jenny and the Queen of Light, as well as in episodes of several television series, including The Zack Files.
He voiced the title character in the series Mattimeo: A Tale of Redwall, which aired in 2000, starred as Lucas Randall in the television series Strange Days at Blake Holsey High, which began airing in 2002 and ended in 2006, and had a role in the 2005 film The Prize Winner of Defiance, Ohio, which starred Julianne Moore. Seater then appeared in the recurring role of Owen on the series ReGenesis, which ended after its fourth season.
Seater garnered wider public recognition for his role as Derek Venturi on the popular Canadian family sitcom, Life with Derek, which was a hit with teens. Seater starred on the series from its premiere in 2005 to its finale in 2009 after four seasons.
Before the show's cancellation, Seater had written a script for an episode of the fourth season with his brother, Graham.
In 2009, he played a character in the Degrassi movie Degrassi Goes Hollywood and in 2010 he starred in the CW series 18 to Life. He also plays the recurring role of murderer and criminal mastermind James Gillies in the Canadian series Murdoch Mysteries.
Seater also had a recurring role on the Canadian TV series Bomb Girls.
In 2015, Seater wrote and directed the feature film People Hold On, which was nominated for a Canadian Screen Award for Best Original Song (by Noah Reid). The film premiered at the Cinefest Sudbury International Film Festival in 2015. It received mixed reviews from critics.
In 2016, Seater wrote and directed his second feature film, Sadie's Last Days on Earth, which was produced by Shaftesbury Films. It was released on December 9, 2016 in theatres. It received mixed reviews from critics, scoring a 40% on Rotten Tomatoes.
Seater is part of the Stand up! Against Bullying campaign with his fellow co-star Ashley Leggat.