It originates from the lateral supracondylar ridge of the humerus, from the lateral intermuscular septum, and by a few fibers from the epicondyle of the humerus. The fibers end at the upper third of the forearm in a flat tendon which runs along the lateral border of the radius, beneath the Abductor pollicis longus and Extensor pollicis brevis; it passes beneath the dorsal carpal ligament, where it lies in a groove on the back of the radius common to it and the Extensor carpi radialis brevis, immediately behind the styloid process (refers to a slender, pillar-like protrusion usually serving as  points of attachment for muscles).

The Extensor Carpi Radialis Longus muscle inserts itself into the dorsal side of the base of the index finger bone ( 2nd metacarpal). It is innervated by the radial nerve. Studies have shown that the Extensor Carpi Radialis Longus receives innervation from the nerves and neurons which arises in the C6 and C7 levels of spinal vertebrae, though the radial nerve does contribute to levels C5, C6, C7, C8, and T1.

The main function and action of this muscle is to allow extensor at the wrist joint, and to abduct the hand at the wrist. The other four major extensor muscles of the superficial layer of the posterior compartment are the Extensor Digitorum, the Extensor Carpi Radialis Brevis, The Extensor Carpi Ulnaris, and the Extensor Digiti Minimi. All of these other four major extensor muscles are innervated by the posterior interosseous branch of the radial nerve.