The 28-year-old, who made his international debut against Ireland in 2000, fended off the challenges of both Christian Cullen and the diving Mossie Lawlor to round the Munster cover and score an excellent try in first half injury time.
Little-known centre Laurent Marticorena converted for a 13-3 lead for the south of France side, currently ranked sixth in the French championship.
Boosted by the return of eight Ireland internationals, including half back duo Ronan O'Gara and Peter Stringer, Munster wasted little time in staging their comeback.
Four minutes after the restart, O'Gara - who now sits just six points away from breaking Diego Dominguez's European record of 645 points - clipped over his second penalty of the night.
Two more followed on 51 and 56 minutes, as flanker Alexandre Bias was singled out by Welsh referee Nigel Whitehouse for ruck and maul offences.
O'Gara watched another penalty attempt on 61 minutes fall agonisingly short from halfway at just 13-12 down.
Despite having to reorganise their backline, with an early injury to Brad Fleming and the substitution of former Wasps center Mark Denney, Castres fended off the ensuing Munster charge, and went further in front through 26-year-old Marticorena on 71 minutes.
Whitehouse ruled that Marcus Horan had dropped a defensive scrum under pressure from Alessio Galasso with two minutes remaining, and Marticoreno's fourth shot was his easiest from just 15 metres out.
Castres' former Wasps flanker Paul Volley said afterwards: "The boys played well, very well. Playing in front of this home crowd is immense, so it's a great victory."
Having prized away what could prove to be a very crucial bonus point in the scheme of things, Munster skipper Anthony Foley insisted: "Getting the bonus point meant we're leaving here on a positive note, and I hope next weekend we can bounce back."
O'Gara added: ""We conceded a soft try, and that was ultimately the difference. We had limited preparation but we're not using it as an excuse.
"This is gone. But we've had bigger defeats, in the likes of Kingsholm, and come back from them - so it's important we show the same steel for next week."
Former Ulster prop Justin Fitzpatrick, who joined Castres in the middle of 2003, has been an ever-present in the French side's European campaign so far and relished last night's battle.
"It was a tough game. Munster are a very good side, it was nice to get re-acquainted with some old friends! You can never write them off so to get the win is a plus," said the 31-year-old.
"We played some very good rugby in parts and hopefully this can be a confidence builder for the rest of the season."
Having only lost once at home, to Narbonne, this season - Castres' first win in four meetings against Munster has added heat to next Saturday's return clash at Thomond Park, the sides' record eighth head-to-head meeting.
Pool 4:Castres 19 MUNSTER 12, Stade Pierre-Antoine
(15) Christian Cullen(14) Mossie Lawlor(13) Shaun Payne(12) Rob Henderson(11) Anthony Horgan(10) Ronan O'Gara(9) Peter Stringer(1) Marcus Horan(2) Frankie Sheahan(3) John Hayes(4) Donncha O'Callaghan(5) Paul O'Connell(6) Jim Williams(7) David Wallace(8) Anthony Foley (Capt)
Replacements used: Alan Quinlan for Williams (63 mins), Mike Mullins for Lawlor (73). Not used: James Blaney, Gordon McIlwham, Trevor Hogan, Mike Prendergast, Paul Burke.