[3] On 32 March 1943 Williams transferred to the Royal Australian Navy where he served as a telegrapher on the corvette HMAS Kiama.

[6] On 25 December the crew was recalled from leave to go to the assistance of the liberty ship SS Robert J. Walker, which had been torpedoed by German submarine U-862.

[7] Kiama was assigned to anti-submarine patrols near Sydney for the final days of 1945 before a month-long refit in Adelaide starting on 3 January 1945.

[5] After the refit Kiama was assigned to Fremantle for two months of anti-submarine warfare exercises with the United States Navy, before returning to New Guinea on 7 May 1945.

[5] In July, Kiama transported Prince Henry, Duke of Gloucester from New Guinea to the Solomon Islands, then spent the rest of the month moving troops and military cargo between these two locations.

[9] Fos Williams was discharged from service on HMAS Kiama on 6 June 1946, nine months after the conclusion of World War II.

[2] Subsequently, Williams became the 609th player selected to play for club in the SANFL making his league debut on 20 July 1946.

West Adelaide looked likely to repeat their success of the previous season but would fall four points short to eventual premiers Norwood in the year's preliminary final.

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.

[10][11] At the end of 1949 Fos Williams was approached by representatives of clubs from Western Australia and Victoria, particularly Geelong and Subiaco, in attempts to lure him away from West Adelaide.

[14][15] 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.

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 five consecutive from 1954 to 1958 and finished second to West Torrens in 1953.

After 1950, Port Adelaide only missed the Grand Final under Fos Williams guidance when they lost the preliminary to Norwood by eleven points in 1952.

[16] In 1953, Fos Williams returned Port Adelaide to the Grand Final, a stage where the club would remain for seven consecutive seasons.

In 1956, Fos Williams again defeating his old club West Adelaide, this time by 16 points in the 1956 SANFL Grand Final.

Unfortunately for both South Adelaide and Williams he could do little with the underperforming Panthers and the club finished seventh with only three wins in his sole season in charge.

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.

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.

In early July, Fos Williams was named for the first time in the South Australian team for the upcoming 1947 Hobart Carnival at the expense of an injured Jack Oatey.

"If I could create an image with words of a red guernsey hell-bent on getting the front position, disregarding personal safety, attacking the ball, swooping up opponents with speed and determination so that nothing was left undone which could have been done, then you would have the picture of the crusade which has earned this state and SA football prestige which I doubt has existed for a long time – if ever before."

Football historian John Devaney wrote of Williams that "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.

Australian Football Hall of Fame legend Barrie Robran said of Fos Williams that "He is a truly great man.