Williams began his league career in 1946, aged 24 with West Adelaide. He was the 609th player selected to play for West and made his league debut on 20 July 1946. Williams played 54 games for West Adelaide, including playing as a rover in their victorious 1947 Grand Final against Norwood at the Adelaide Oval. Williams first gained state selection for South Australia while with West Adelaide and also kicked 112 goals for the club until the end of 1949.
Fos Williams left West Adelaide to become captain-coach of Port Adelaide in 1950. In his first year at Port he won the club's best and fairest award and led the team to a preliminary final, where they were however well-beaten by Glenelg. In 1951 he won his second SANFL premiership as a player and the first of a then-record nine as a coach when he led the Magpies to an eleven point win over North Adelaide in the Grand Final after losing only one game on a Thebarton Oval mudheap all season. Under the coaching of Williams from 1950 until 1958 Port returned to a position of power in the league and along with the 1951 win they won 5 in a row from 1954–58 and finished second to West Torrens in 1953. The Magpies only missed the Grand Final when they lost the preliminary to Norwood by eleven points in 1952. During their run from 54-58 under Williams Port defeated Norwood in 1955 and 1957 while the 1954, 1956 and 1958 wins were against his old side West Adelaide. After leaving Alberton at the end of 1958, Williams influence at the Magpies was still evident in his three-year absence with the team's style of play changing little under new coach (and Williams' old team mate) Geoff Motley. Motley took over as captain-coach and Port would win a sixth successive flag in 1959 and finish third in both 1960 and 1961 before Williams returned in 1962.
During his time as captain-coach of the Magpies, Fos Williams wrote the Port Adelaide Football Club Creed.
After retiring from playing following the 1958 premiership, Williams coached South Adelaide in 1960. Unfortunately Williams could do little with the underperforming Panthers and the club finished seventh with only three wins in his sole season in charge. Williams returned to Alberton in 1962 as non-playing coach taking over from Geof Motley, who stayed on as team captain. Williams once again led Port Adelaide to premierships in 1962, 1963 and 1965. From the time he returned in 1962 until 1968 Port Adelaide appeared in every SANFL Grand Final, losing the last three in a row (1966–68) to the Jack Oatey-coached Sturt, which would go on to win five in a row (adding 1969 and 1970) equalling the record of Williams and Port from 1954-58.
The Port Adelaide Football Club's creed was written and spoken for the first time in 1962 by Fos Williams.
"We, the Players and Management of the Port Adelaide Football Club, accept the heritage which players and administrators have passed down to us; in doing so we do not intend to rest in idleness but shall strive with all our power to further this Club's unexcelled achievements. To do this we believe there is a great merit and noble achievements in winning a premiership.
To be successful, each of us must be active, aggressive and devoted to this cause. We agree that success is well within our reach and have confidence that each member of both the team and management will suffer personal sacrifices for the common end.
Also we know that, should we after striving to our utmost and giving our everything, still not be successful, our efforts will become a further part of this Club's enviable tradition.
Finally, we concede that there can be honour in defeat, but to each of us, honourable defeat of our Club and guernsey can only come after human endeavour on the playing field is completely exhausted."
Fos Williams continued to coach the Magpies until the end of the 1973 season when they finished fifth. During his 21 seasons at the helm of the Magpies, Port Adelaide had competed in twenty finals series (missing only in 1969), sixteen Grand Finals and had won nine premierships. At the time this was the record for coaching in the SANFL. It was the man who replaced Williams as Magpies coach for 1974, John Cahill, who would go on to break Williams' record of 9 premierhsips when he won ten, all with Port Adelaide.
Williams returned to West Adelaide as coach in 1974 but the side finished with its third "wooden spoon" in a row. Williams recruited former Port Adelaide rover Trevor Grimwood in 1975 and the Bloods improved, making their first finals series since 1969 by finishing fifth in 1976 and improved again to reach the finals again in 1977, ultimately finishing third after losing to Glenelg in the Preliminary Final at Football Park. 1977 also saw Grimwood win the Magarey Medal. 1977 was the 23rd and last SANFL Finals series to feature Fos Williams as a coach.
After finishing with a 14-8 record in 1977, West Adelaide slumped in what was Fos Williams' last year as an SANFL league coach in 1978. The Bloods finished with only five wins and a draw and ninth place; however one of the wins was the only loss inflicted upon Sturt prior to the Grand Final. Following this disappointing result Williams retired from coaching.
During his career as a player Fos Williams represented South Australia on 34 occasions in Interstate matches, winning a Simpson Medal for best on ground in the 1955 WA v SA match played in Perth. He was captain of the state from 1954-1958. Williams was also the coach of the South Australian team from 1955–58, from 1960–66 and again in 1968 and 1969. During his time as state coach, Williams was instrumental in instilling the belief in his players that their usual tormentors, Victoria, were no better than they were and it all culminated in the 1963 with a seven-point win over the Big V at the Melbourne Cricket Ground (MCG), their first win in Melbourne since 1926.
Fos Williams was named in the "Sporting Life" Team of the Year in both 1950 and 1951. On both occasions he was selected in the Forward pocket.
One commentator wrote "Perhaps more than any other single individual, Fos Williams was responsible for catapulting South Australian football out of its predominantly casual, laissez faire mentality into the same kind of professional, brutally expedient, 'win at all costs' mindset as prevailed across the border in Victoria."