The giant made his debut for the Irish province during the 1998-99 season in the old Ireland inter-provincial championship.
He has since gone on to win 65 Ireland caps and was a member of the 2005 Lions touring party to New Zealand.
During that tour Horgan played in eight games including all three clashes with the All Blacks.
Horgan never thought he would go on to enjoy such a distinguished career which has also included Magners League titles, a Heineken Cup and a RBS 6 Nations Grand Slam.
"My senior debut was against Ulster in Donnybrook and we ended up losing the game," Horgan told the Irish Independent.
"It's hazy enough, I was playing in the centre that day, Andy Ward was playing for Ulster.
"When I came in to Leinster, I was playing with guys I would have watched on TV and some of them would have been heroes at the time, it felt weird and took quite a while to settle in.
"Now guys are in the team for a while, but back then you were in your club and then you came into Leinster. But that debut was a huge deal.
"I never thought I'd get this far. It shows my age but when I started it was only about four, five or six games a season, I would have had to play for about 40 years to make that ... but it's very nice, a personal milestone, but I think certainly my focus this week is on this game and Munster, and it really does focus the mind."
Leinster face their Irish rivals in a crunch Magners League clash at Thomand Park. Munster sit at the top of the league table and have virtually secured their place in the play-offs while Leinster are one of three teams with 55 points leading the chase.