[3] Pisidia longicornis is found in the north-eastern Atlantic Ocean from Norway to Angola, and throughout the Mediterranean Sea.

[2] It lives under boulders and among bryozoan turfs,[3] and may occasionally be found in fish markets among mussels and oysters.

Individuals from the intertidal zone tend to be larger than those from deeper water, which may be related to the greater churning of the sediment by the tide.

[5] Females bear eggs from March to August around the British Isles, and from February to September in the Mediterranean Sea.

The specific epithet longicornis is difficult to explain, it being Latin for "long-horned", from longus and cornu; it may refer to the long chelipeds (claw-bearing appendages).