But with their rivals Dartford and Gosport Borough also losing they remain in the fifth and final play-off place on a day when victory would have lifted them to fourth.
And City boss Steve Tully was not too despondent.
He said: "It was just one of those days. We had too many key players not at their best today.
"I think we had 90 per cent of the possession but we got what we deserved. It had to happen sooner and later and now we have to go on another run. Results have gone with us so we stay where we are.
"We are going to get lots of different puzzles to solve between now and the end of the season and we could not solve it today. They set up 4-5-1 and got bodies behind the ball. We had a good spell either side of half time and had some half chances.
"Mattt Jay, who had a good game, had a great chance near the end but could not get his shot away."
City were unchanged for the third game running, but began badly as the home took an early lead after 13 minutes with Hemel debutant Ricky Wellard putting Graeme Montgomery through and he finished well past Martin Rice to put The Tudors 1-0 up.
Rice then had to tip over a Nikolay Todorov effort as Hemel pressed forward.
And the pressure finally paid off just before the interval when Shane White netted.
City had a let off at the start of the second half when Nikolay Todorov hit the woodwork with James Potton's follow up effort blocked before City managed to clear the danger.
But the home side regained the lead 14 minutes from time when a shot by substitute Ibra Sekajja was blocked and Montgomery was on on hand to head in the rebound and hard though City tried they were unable to find an equaliser.
They are next in action at home to Bath City on Tuesday evening
Hemel Hempstead Town: Butler; Herd (capt),Kelly, Bailey-Dennis, Thorne, Hickford, Wellard, Richens, Todorov, Potton, Montgomery.
Subs: Mitchel-King, Sekajja, Parrott, Fitzgerald, Spring
Truro City: Rice; White, Pugh (capt), Bentley, Riley-Lowe; Knowles, Green, Cooke, Brett, Vassell, Jay.
Subs: Ash, Duff, Dawson, Duff, Adelsbury.
Goals: Montgomery (13) 1-0, White (44) 1-1, Montgomery (76) 2-1.
Read more: http://www.westbriton.co.uk/Hemel-Hempstead-Town-v-Truro-City/story-28910186-detail/story.html#ixzz42tVllHdu