Paul Sackey, Tom Voyce and Josh Lewsey all crossed for the hosts, with England fly-half Danny Cipriani adding the remaining 10 points courtesy of a brace of conversions and penalties.
The win was an impressive response to what has been a disappointing start to the season for the current English Champions. Since winning the Premiership crown at Twickenham last May, Wasps have lost five of their first six competitive fixtures of their new league and cup campaign.
"All in all I was really pleased," said McGeechan, who has already stated that the Heineken Cup will form a part of his selection process for next summer's Lions tour to South Africa.
"I thought we started well, we went back to a lot of things we do well and I think that showed.
"It was a good defensive performance and offensively we were very slick. We wanted to keep the tempo up and Castres wanted to slow the tempo down.
"Raf (Ibanez) said in the week that the experienced players had to take a lead and show up for real and I thought all of them did that. Our experienced players delivered big time.
"It was obvious that it meant a lot to everybody. Honesty has been what this club is about and that came out today. There was a lot of work went in and we got the benefits."
McGeechan had promised in his programme notes that the start of the Heineken Cup would provide the ideal platform to kick start their year, and his side clearly took note as they opened up a 20-3 lead by the half-time interval.
The English champions started the scoring with seven minutes on the clock thanks to a fine piece of individual skill from Riki Flutey. The New Zealand-born centre, who was named in England's Elite Player Squad after qualifying on residency grounds, put Wasps on the front foot having picked up loose ball mid-way inside his own half. Flutey found a hole in the disorganised Castres defence before sending a beautifully-weighted grubber kick towards the onrushing Paul Sackey on the right wing. Sackey again showed that he needs little help in locating the tryline as he powered over just five metres in from touch, taking three French defenders with him.
After expertly converting with his left boot to give the hosts a 7-0 lead, Cipriani was heavily involved in Wasps' next scoring move. The 20-year-old fly-half's weaving break took play to deep within the Castres 22 and, although the visitors' scramble defence held firm, they did so illegally to hand Cipriani a straightforward penalty attempt.
Cipriani duly made it two successful kicks from two attempts with 14 minutes gone but he then struck the left-hand post with a 35-metre penalty as the game approached the end of the first quarter.
Wasps were soon back on the scoreboard, however, as Tom Voyce crossed for his side's second try of the afternoon. The former Bath winger made the most of some lax Castres tackling to move Wasps even further in front after receiving the ball 25 metres out.
Cipriani added the extras from underneath the posts as McGeechan's men moved 17-0 ahead with 22 minutes played.
Cameron McIntyre finally put points on the board for Castres as he made no mistake with his first penalty attempt from 40 metres out and 10 metres in from the right touchline.
Wasps continued to dominate proceedings with Cipriani given another shot at goal with the game half an hour old. The score stayed at 17-3, though, as the England playmaker sent his 25-metre attempt wide of the near post from 15 metres in from the right touchline.
Castres were then dealt a double blow four minutes later as openside flanker Darron Nell was ordered to spend 10 minutes in the sin bin, with Cipriani kicking the resulting penalty to push Wasps 17 points clear.
McIntyre's second long-range penalty narrowed the gap with five minutes of the second period played before Wasps reasserted their authority 13 minutes later. Cipriani and Lewsey put their mid-week training bust up behind them as they combined for Wasps' third try of the first hour.
Cipriani just failed to gather his own chip ahead on the edge of the Castres 22 but he did enough to force the ball loose when tackling full back Thomas Bouquie, with Lewsey the man to benefit. The World Cup winner's impressive reactions saw him pick up and race 20 metres to the Castres line, much to the delight of a 8,800-strong crowd in High Wycombe.
Cipriani failed to make it a seven-point score as he sent his conversion wide of the uprights from five metres in from the right touchline but Wasps were once again more than two scores clear at 25-6.
Lewsey became the second player to receive a yellow card with just over an hour played but Castres were unable to take advantage as they failed to add to their tally during the Wasps centre's absence.
The French side did claim their second try of the match with 74 minutes played, however, as skipper Lionel Nallet forced his way over from close-range after a period of concerted pressure. McIntyre missed with the boot for the first time in three attempts but it made little difference to the end result.
Castres remained on the front foot for the final few minutes but it was Wasps who took the points to move into second place in the Pool 2 table ahead of their trip to Dublin next weekend.
