The winger’s brace helped England on their way to a dominant victory, registering 31 shots on the visitors’ goal across the 90 minutes.
The Lionesses knew they would claim the trophy if they avoided defeat at Ashton Gate.
Kelly, starting alongside City team-mate Alex Greenwood, grabbed the opener when she volleyed home from close-range after being set up by Lauren James’ searching run to the by-line.
That strike set the tone for the European champions, and they could have been two up before the half hour when Millie Bright headed over Greenwood’s dangerous delivery.
But the hosts didn’t have to wait long for a second with captain Leah Williamson doubling the advantage on the stroke of half-time.
Lauren Hemp was introduced at the break as the Lionesses looked to turn the screw, and the winger would play an important role in her nation’s third goal of the evening.
It arrived soon after the restart, with the substitute threading through Ella Toone, who in turn squared for Kelly to tap home before the 50-minute mark.
The strike - her third of the tournament - ensured she would finish as top scorer, but City’s number nine almost completed her hat-trick minutes later when a vicious effort from the right clipped the top of the crossbar.
A fourth would follow as the game approached the 80-minute mark though, with Julie Biesmans putting the ball in her own net under pressure from Rachel Daly.
City youngster Jess Park was introduced soon after as England’s stranglehold continued, with Lucy Bronze heading home a fifth in the 88th minute.
But the Belgians - who could have won the tournament with victory - would reply in spectacular fashion as the game ticked over into stoppage time, Elena Dhont rifling home an exquisite drive from distance.
It was the hosts who would have the final say though, with Williamson calmly slotting home her second and the Lionesses’ sixth to round off an exciting evening of action.
It means England’s unbeaten run has now been extended to an astonishing 29 matches ahead of this summer’s World Cup in Australia and New Zealand.
Elsewhere, Yui Hasegawa was on target for Japan as they rounded off their SheBelieves Cup campaign with a 3-0 win over Canada.
The midfielder doubled her nation’s lead on the stroke of half-time when she found the net from the penalty spot, building on Kiko Seike’s earlier strike.
And the scoring was complete with 13 to play when Jun Endo added a third to hand the Japanese their first win of the tournament in their final match.
With Brazil also losing to the United States, it means Hasegawa and co. finish the SheBelieves Cup in second place.
Elsewhere, Mary Fowler was a late substitute as Australia claimed the Cup of Nations with a 3-0 win over Jamaica.
The young midfielder was introduced late on to help the Matildas over the line, with City team-mates Hayley Raso and Alanna Kennedy both rested.
Australia, host nations at this summer’s World Cup, were in control throughout, with goals from Katrina Gorry, Alex Chidiac and Caitlin Foord earning them victory.
