The Redskins lost their final eight games a year after winning the NFC East title.
Shanahan made the controversial decision to bench starting quarterback Robert Griffin III with three games to go in the season. He said the move was to keep Griffin healthy for the off-season and had the support of team owner Daniel Snyder.
That didn't stop Snyder from making the coaching change.
"Redskins fans deserve a better result," Snyder said. "We thank Mike for his efforts on behalf of the Redskins.
"We will focus on what it takes to build a winning team and my pledge to this organization and to this community is to continue to commit the resources and talent necessary to put this team back in the playoffs."
Shanahan said he felt the team was in a better competitive position -- mainly because of salary cap issues -- than when he took over four seasons ago.
He started his statement by thanking the Redskins' fans, Snyder for the "opportunity to be here" and the Washington players. He didn't address reasons that led to his being fired, however.
Shanahan, who previously coached at the Los Angeles Raiders and Denver Broncos, was hired Jan. 5, 2010, as the Redskins' head coach. He was the sixth full-time head coach for Washington since Snyder bought the team in 1999. At 64 games, he tied Joe Gibbs as the longest-tenured coach under Snyder.
The Redskins had just one winning season under Shanahan, going 10-6 in 2012. The team was 24-40 over the last four seasons, giving Shanahan an overall coaching record of 170-137.