The 2013 Tour champion, who will feature among the top favourites for the 4-26 July race, made his move 2.5km from the finish in the final climb to Valfrejus and never looked back.
The Team Sky rider quickly swallowed up Stephen Cummings (MTN Qhubeka), who had spent the day in the breakaway, and distanced yellow jersey holder Tejay van Garderen (BMC).
Froome, who also prevailed in Saturday’s uphill finish in St Gervais, beat Simon Yates (Orica-GreenEDGE), to take the day’s laurels with an 18 second advantage.
At the end of Sunday's 156.5km trek from St Gervais, Van Garderen was fourth, behind Rui Costa (Lampre-Merida), both 18 seconds behind Froome.
Overall, Froome beat Van Garderen by 10 seconds with Costa in third place, one minute 16 seconds off the pace.
"I can't believe it. I couldn't expect to go any better today. The legs were tired after yesterday,” Froome said. “My whole team suffered today but everyone gave everything as we had the yellow jersey in sight.
“Ian Stannard rode alone at the front of the bunch for 100 kilometres. The rest of the team was fantastic until the last moment when I could attack.
“Every second counted at the end. I was excited all the way to the finish. That's what bike racing is about. We were off the back a little bit after the team time trial and we had to regain time.
“It's amazing to finish off a way like this. That's gonna be a day I'll remember for the rest of my life. The Dauphiné is a special race for me. I'm pretty happy with how I go at the moment. This race helps building the team.
“The Tour de France is my big objective. The team is ready. I'm almost ready. It's less than three weeks away. I'm looking forward to it.”
Among Froome's main Tour de France rivals, defending Tour champion Vincenzo Nibali looked far from his best.
The Astana team leader took 12th place, four minutes 32 seconds behind Froome after being unable to follow the best in the climbs on Saturday and Sunday as well as in Thursday's fifth stage to Pra Loup.
Nibali, however, also looked under-par last year in the Dauphiné before going on to smash his rivals on the Tour.
Alejandro Valverde (Movistar) finished ninth, 3min 12sec adrift. Exciting prospects Romain Bardet (AG2R) and Yates were sixth and fifth respectively.
Other Tour contenders were not racing the Dauphine, the most prestigious warm-up event for the three-week extravaganza.
Leading favourite Nairo Quintana (Movistar) and double Tour winner Alberto Contador (Tinkoff-Saxo) who won last month's Giro d'Italia, will take part in La Route du Sud in France next week.
Thibaut Pinot (FDJ), third in last year's Tour, is riding the Tour de Suisse where he is among the leaders after two stages.
Stage 8: 156.5km, Saint-Gervais Mont Blanc - Modane Valfréjus
1 Christopher Froome (GBR) Sky 3hr 59min 27sec
2 Simon Yates (GBR) Orica-GreenEDGE 0:00:18
3 Rui Costa (POR) Lampre-Merida
4 Tejay Van Garderen (USA) BMC
5 Joaquim Rodriguez (ESP) Katusha 0:00:28
6 Romain Bardet (FRA) AG2R
7 Daniel Martin (IRL) Cannondale-Garmin 0:00:31
8 Wouter Poels (NED) Sky 0:00:44
9 Pierre Rolland (FRA) Europcar
10 Benat Intxausti (ESP) Movistar
General classification
1 Christopher Froome (GBR) Sky 30hr 59min 2sec
2 Tejay Van Garderen (USA) BMC 0:00:10
3 Rui Costa (POR) Lampre-Merida 0:01:16
4 Benat Intxausti (ESP) Movistar 0:01:21
5 Simon Yates (GBR) Orica-GreenEDGE 0:01:33
6 Romain Bardet (FRA) AG2R 0:02:05
7 Daniel Martin (IRL) Cannondale-Garmin 0:02:52
8 Joaquim Rodriguez (ESP) Katusha 0:03:06
9 Alejandro Valverde (ESP) Movistar 0:03:12
10 Andrew Talansky (USA) Cannondale-Garmin 0:04:17
