Wales' coach Warren Gatland made 11 changes to the side that beat Samoa last week, and they took control of the game in the final 30 minutes after Namibia's Raoul Larson was sent to the sin bin by referee Steve Walsh for repeated infringing.
Twenty-year-old midfield back Scott Williams bagged the first try in the eighth minute of the game, after veteran first five-eighths Stephen Jones put the first points on the board for Wales with an early penalty goal.
When Williams coasted through for his second try early in the second spell, Wales had secured the all-important bonus point for scoring four tries.
He later became only the fourth Welsh player to register a hat-trick in a World Cup game, when he dotted down on the 70th-minute mark and Wales piled on 12 tries in all.
Jones who was playing his 101st test, overtook former captain Gareth Thomas as the most capped Welshman, during an audition to prove he should be the side's first-choice pivot ahead of youngster Rhys Priestland.
The 33-year-old also passed 900 test points for Wales, coupled with the 53 he has amassed for the Lions, he is now closing in on Andrew Mehrtens' 967 test match points, which would see him leapfrog the former All Black into sixth spot on the all-time list.
Welsh prop Gethin Jenkins brought a smile to Gatland's face with a bullocking try as the giant prop barged through the defence from 30 metres. It was an important moment for the front-rower, who has struggled with injuries during the year. The try confirmed he is back to full fitness.
Heinz Koll grabbed his second try of the tournament as the bustling Namibian lock found his way to the line in the 53rd minute.
Theuns Kotze had a couple of chances to put Namibia on the board during the first 40 minutes, but his penalty attempts were unsuccessful.
The match marked the end of Namibia's World Cup campaign, which also featured an 87-0 loss to South Africa, and was always going to be a challenging ask considering they had a tricky pool.
Aside from Wales and South Africa, Namibia also suffered defeats to Fiji and Samoa, but have shown throughout the tournament that they can be competitive. There have been calls for them to get more regular test matches in future.
Wales: 81 (Scott Williams 3, George North 2, Aled Brew, Lee Byrne, Jonathan Davies, Toby Faletau, Gethin Jenkins, Alun Wyn Jones, Lloyd Williams tries Stephen Jones 6, Rhys Priestland 3 cons S Jones pen)
Namibia: 7 (Heinz Koll try Theuns Kotze con)
Half Time: 22-0
