It was only in November that the ex-Batley Bulldogs prop agreed to extend for 2018 after a successful first season in Super League.
However, the club have announced Hirst, 30, will now stay on until the end of 2019.
He made 23 appearances for Wakefield last term after making the step-up from the Championship and has continued his good form this time around.
“I am thrilled to get the deal over the line,” said former Featherstone Rovers and Dewsbury Rams front-row Hirst.
“After a successful season last year not only with the club but on a personal level it is nice to have a bit of security of a new deal and to be able to concentrate on this season.
“It was a huge gamble for both me and the club to make the move to Super League, but I am thoroughly enjoying playing for Wakefield.
“It is a great club with a fantastic support network. The more experience I have in a full-time environment the more it will help me improve as a player.”
Head coach Chris Chester, who yesterday saw Hirst help his side to a fourth win from as many Super League games with a 22-4 win over Huddersfield Giants, said: “It is great to have Keegan on board until 2019.
“He’s a fantastic player to have in our ranks and not only does he provide excellent competition to our pack but he is also a huge character to have in the squad.
“It’s fantastic that as a club we are able to secure players so early on in the season, so that our whole focus can be on the games ahead.”