The flanker joined the Exiles from Worcester in 2013 and is closing in on 100 appearances while he has also gone on to win 17 caps for Scotland, featuring at the 2015 World Cup.
Having played a leading role in helping Irish win promotion back to the top flight in 2016-17, Cowan has continued his form into the new season as Nick Kennedy's men aim for survival.
The 31-year-old admits he is delighted to have settled his future with a club he is confident is heading in the right direction.
"I'm stoked to have extended my contract with London Irish," said Cowan. "London Irish is a club that has had faith in me from day one and has always allowed me to play my natural game, culminating in international recognition.
"I've witnessed first-hand the exciting changes that have taken place at the club since making the move to Irish four years ago.
"When I finally hang up my boots, I want to know that I've done everything I can to help the club be in a better place than when I joined."