The Harlequins prop will overtake David Campese as the second most capped player in rugby history when England tackle unbeaten RBS 6 Nations title rivals Ireland at Lansdowne Road on Sunday.
But in addition to making his 102nd Test match appearance, Leonard will also become the first post-war England player to win four Grand Slams if Ireland are defeated.
The 34-year-old helped his country achieve championship clean sweeps - all under Will Carling's captaincy - in 1991, 1992 and 1995.
He has also been on the receiving end though, featuring in England's three final-game flops - 1999, 2000 and 2001 - during Clive Woodward's coaching reign.
"What people forget is that losing a Test match is painful. It doesn't matter whether it is for a Grand Slam or not - it hurts a lot because of your national pride," Leonard said.
"I have won Slams, and I've lost Slams, but one thing is for certain, we are a stronger team because of those set-backs."