Slack's family migrated from the Windward Islands to High Wycombe in Buckinghamshire, England, when he was 11 years old.

He was a left-handed opener and played county cricket for Middlesex between 1977 and 1988. He also played for his native Windward Islands in their opening seasons of the West Indies domestic competition in 1981–82 and 1982–83, at the request of Michael Findlay, the former West Indian wicket-keeper to whom he was distantly related.

He spent time playing cricket for various local sides, progressing into the Buckinghamshire team in 1976, at the age of 21, becoming the minor county's leading run-scorer for the season, with 748. The Middlesex coach, Don Bennett, marked him as first-class county material, and he was signed by them the next year.

Slack made his Middlesex debut in 1977, but failed to establish himself in the side, in part because he played out of position, in the middle order.

When Mike Brearley, Middlesex captain and batsman, was recalled to the England side in 1981, Slack was called on to replace Brearley and open the batting for Middlesex against Kent at Lord's; he scored his maiden first-class century, 181 not out. The unbroken stand of 367 by Slack and his partner (Graham Barlow) was a Middlesex record. In the following game he made 248 not out, against Worcestershire; both were scored in the second innings. He finished the season with 1,303 Championship runs at 48.25. This auspicious opening with Barlow foreshadowed their profitable partnership, one of the best opening pairs in the County Championship circuit of that era, until Barlow retired in 1986, which along with domestic problems affected Slack's form. Slack completed 1,000 runs in a season eight times. In 1985 he bettered his effort of four years earlier by making 1,900 runs at 54.28, and was rewarded with a tour of Sri Lanka with the England B side.

Simon Hughes wrote of him in A Lot of Hard Yakka: “Wilf Slack, a reserved Windward Islander who never betrayed any nerves despite the daily task of standing up to some of the fastest bowlers in the world, rarely said anything when he came back into the dressing-room. He’d sit down, quietly unbuckle his pads, and carefully lay them to rest in his case, then stare glumly into space for a while. He was deeply religious, which was possibly an explanation for such contemplation.” Besides holding nearly 200 catches, many at bat-pad, he was always eager to bowl his military-medium pace, especially in limited-overs matches.^[6]

Whilst in Sri Lanka, Slack was rushed to the West Indies during England's 1985–86 tour to replace the injured Mike Gatting. Slack made just two runs in the two innings of his Test debut at Port-of-Spain. As a result he was dropped for the next two Tests, but returned in the Fifth Test to make a tidy 52, partnering Graham Gooch in an opening stand of 127.

His third and final Test came at Headingley against India in 1986. He again failed to impress but had a successful county season by topping 1,000 runs once more. This won him a place in the England squad for the Ashes tour of Australia in 1986–87. He did not play a Test in the series, and was never again picked for England.