Wishart blasted an unbeaten 172, the highest total by a Zimbabwe batsman in one-day internationals but short of Gary Kirsten's 188 not out for South Africa against the United Arab Emirates in the 1996 World Cup. Pakistan's Saeed Anwar holds the world record one-day score of 194.
Wishart, who struck three sixes and 18 fours in 149 balls, shared an opening stand of 107 with Mark Vermeulen, who made 39 before falling to the left-arm spin of Lennie Louw in the Group A opener.
The 43-year-old Louw, the oldest player in the tournament, produced a juggling catch off his own bowling to claim Namibia's first ever World Cup wicket.
Andy Flower, who issued a hard-hitting statement with team mate Henry Olonga before the start of the match criticising Zimbabwe's record on human rights and democracy, scored 39 before being removed by Jan-Berry Burger's occasional leg-spin.
Flower, arguably Zimbabwe's greatest player and among the top batsmen in the world, had walked out on to the field wearing a black arm band in what is expected to be his last major international event for Zimbabwe.
But Wishart and Grant Flower (78 not out) lifted Zimbabwe past 300 with an unbroken third-wicket stand of 166, cashing in on a good batting pitch and an inexperienced Namibia attack.
The slightly-built Bjorn Kotze, nicknamed 'Bones', was hammered for 75 runs in his 10 overs while fellow medium pace seamer Louis Burger went for 70 in 10.
The hard-hitting Wishart reached his second century in one-day internationals at a run a ball and then accelerated, a savage pull for four taking him past Andy Flower's 145 against India in Colombo last year, Zimbabwe's previous highest individual score.
Wishart, whose first hundred in one-dayers was 102 against India at the same venue in 1998, reached his 150 by hoisting a towering six over long on off the seam bowling of Gerrie Snyman.
Zimbabwe's innings total was their highest in a World Cup, eclipsing their 312 for four against Sri Lanka at New Plymouth in 1992.
Namibia are playing in their first World Cup after surprisingly finishing runners-up to the Netherlands in the 2001 ICC Trophy in Canada.
