The Welsh athlete and Commonwealth champion finished with a time of 4.02.83 and will be in the scrap for a medal later on Sunday. Defending champion and world record holder Katie Ledecky of the USA clocked a time over four seconds faster than the rest of the field and is seemingly destined for another Gold at a Summer Games.
James Guy also shrugged off the disappointment of finishing sixth in Saturday's 400m freestyle final, as the Briton qualified fifth-fastest for the semi-finals of the men's 200m free.
Carlin eases into Olympic final
Carlin in her first Olympic appearance, showed now signs of nerves starting cautiously as Ledecky raced away from the pack in the first 100m and never looked back.
The 25 year-old Swindon native placed herself into the chasing four and over the last 100m pushed ahead to beat American Leah Smith, Coralie Balmy of France and Canadian Brittany MacLean.
With Ledecky set for Gold, the other two medals should be decided between those four and Australian Jess Ashwood.
It could be an eventful 20 minutes in the Olympic Aquatics Stadium for GB later, as Carlin competes in the final minutes after Adam Peaty aims for the first British male pool gold in 28 years in the 100m breaststroke, UK time 2.53am.
Guy more conservative swim
The defending champion from the World's in Kazan last August, Guy went out with a more conservative approach to his final swim, but looked more comfortable following yesterday morning's sluggish race. With more left in the tank and faster 200m, Guy will still fancy his chnaces of a podium finish.
Guy came second in his heat behind American Conor Dwyer, who himself qualified fastest for 400m free final, but also failed to medal.
Sun Yang of China progresses with the fastest time of 1.45.75 after pipping current 200m butterfly champion from London 2012, Chad le Clos. The South African blitzed into a customary early lead in his heat, but was caught by the surging Yang in the final strokes.
World record holder Paul Biedermann of Germany is just three-one-hundreths of a second slower than the Chinese going into tonight's semis. Olympic champion in the 400m individual medley Kosuke Hagino, qualifies in seventh.
Elsewhere, in the women's 100m breaststroke heats, GB's Chloe Tutton also qualified for the semi-finals and Loughborough's Georgia Davies qualifiied seventh fastest for the 100m backstroke.