Head Coach Nick Walshe's side got off to a strong start, putting their hosts under pressure, but they weren't able to turn the pressure into points as Slade missed two early kicks. The breakthrough eventually came with 12 on the clock, Slade making no mistake with his third attempt to make it 3-0.
The first try of the game came with 23 on the clock, a line-out was well collected by the pack and hooker Cowan-Dickie eventually touched down from the back of the rolling maul. Slade added the extras to give England a 10-0 lead.
It wasn't long before France were on the board though, scrum half Baptiste Serin sent a penalty through the sticks for 10-3 before Slade hit back to make it 13-3 after 27 minutes.
The hosts were then reduced to 14-men after No. 7 Alban Placine was sent to the sin-bin for stamping, but England couldn't capitalise and it stayed 13-3 until the break.
France got the opening score of the second period when Serin added his second penalty to make it 13-6. But England hit back with an excellent team try after 56 minutes. A quick break from wing Anthony Watson freed up Alex Day and Day played the ball inside to Clifford who showed a good burst of pace to go between the sticks.
Slade added three more to make it 23-6 before Clifford scored his second, picking up a loose ball before racing over for his fifth try in two games against France - having scored a hat-trick against them in the Six Nations.
Walshe said: "We're pleased to get the win, but the important thing is that we know we can play better. This is a five game tournament, not a one game tournament. Our focus is now on South Africa, we'll recover from this one and then switch all of our attention to the next one."
Clifford added: "We knew that this was going to be tough, and that's how it proved. The scoreline probably doesn't reflect the match - we were under pressure for long periods and our defence held firm. They were much improved from when we played them in the Six Nations so we're delighted to get the win."
Points: Slade 3P, 3C; Cowan-Dickie 1T, Clifford 2T.
England Under 20 starting XV against France
15 Ollie Devoto (Bath Rugby)
14 Anthony Watson (Bath Rugby)
13 Harry Sloane (Harlequins)
12 Sam Hill (Exeter Chiefs)
11 Ben Howard (Worcester Warriors)
10 Henry Slade (Exeter Chiefs)
9 Callum Braley (Bristol Rugby)
1 Alec Hepburn (London Wasps)
2 Luke Cowan-Dickie (Exeter Chiefs)
3 Scott Wilson (Newcastle Falcons)
4 Elliott Stooke (Gloucester Rugby)
5 Dominic Barrow (Leeds Carnegie)
6 David Sisi (Bath Rugby)
7 Matt Hankin (Saracens)
8 Jack Clifford (Harlequins) (c)
16 Nathan Morris (London Wasps) on for Alec Hepburn 60 mins
17 Danny Hobbs-Awoyemi (Northampton Saints) on for Luke Cowan-Dickie 70 mins
18 Tom Smallbone (London Irish) on for Scott Wilson 73 mins
19 Tom Price (Leicester Tigers) on for Dom Barrow 54 mins
20 Ross Moriarty (Gloucester Rugby) on for David Sisi 63 mins
21 Alex Day (Northampton Saints) on for Callum Braley 52 mins
22 Henry Purdy (Leicester Tigers) on for Harry Sloane 66 mins
23 Jack Nowell (Exeter Chiefs) on for Ben Howard 55 mins