He added the hashtag "#skinnybeast" to his post.
In October, the star shared his health journey on Twitter, posting that he "lost four inches off my chest and six inches of my belly in the last six months."
The quiz master revealed he weighed in at 171 kg (378 pounds) at his heaviest, Hello! reports.
Fans who commented on his Instagram post shared their messages of encouragement.
"Great work Mark," one person commented. "Keep striving for health and fitness buddy! There is no better wealth than health!"
Another wrote: "Keep going pal, it's hard work but it's achievable."
A third said: "Well done achieving that over the last months, being in close proximity to my fridge for most of the waking day has certainly been bad for my waist line."
The Mirror reported in October he shed 20kg, and was inspired to lose weight after being diagnosed with type 2 diabetes.
In 2017, Labbett appeared on UK panel show Loose Women as part of their male Body Stories campaign, where he opened up about his type 2 diabetes diagnosis.
He confessed to eating up to 10 bars of chocolate a day, he said: "I'm not going to complain. I've done the crime so I'll do the time."
However, he also spoke about maintaining a good relationship with his body, simply stating: "I am what I am."
Labbett and his wife Katie split last May after he discovered she was cheating on him, but they have since got back together.
