Williams, who came on as a replacement against the RAF, started in Services’ youth system and then graduated to colts and then the senior side.
He follows in the footsteps of his dad, Russ, who also played for Services and the Navy.
Williams is now hoping to feature in next Saturday’s Army-Navy game in front of 80,000 people at Twickenham.
Meanwhile, Williams’ Services team-mates end their season this Saturday when they travel to Wadebridge.
Services beat the Cornish side at home on the opening day of the season, which gave them great hope for the 2015/16 campaign.
But team manager Kieran Leach said: “Injuries to key players affected the team badly and consequently the season didn’t meet our early promise.
“We have had some great performances though against Kingsbridge, Exeter University and Okehampton twice.”
Services are still suffering with injuries. Props Jase Money and Josh Morrison are out of this weekend’s match along with the entire back three from last Saturday – Ed McGinley, Ben McGowan and Kelli Bainivalu. Alan Bodel, Tom Morgan and Sam Luke come in.
Meanwhile, Devonport Services will play in the South West Ladies’ Under-18 final against Wellington at the Rectory on Sunday (3pm).
For five of the team it will be their last under-18 match and they hope to sign off in style.