The giant wing made his Leinster debut in 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 British & Irish Lions touring party to New Zealand.
During his time at Donnybrook and the RDS, Horgan has racked up 96 Magners League appearances and touched-down 35 times.
"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."
Horgan never thought he would reach anywhere near the milestone he is about to break. But he insists he is just as motivated as ever particularly with their crunch Magners League clash at Thomond Park quickly approaching.
"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."
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