Angrim joined the Ithilien Rangers several months before Faramir assumed the captaincy in July 3006 (see Proof), eventually becoming one of Faramir's lieutenants. He remained a Ranger until his death at the Causeway Forts in March 3019 (see Mercy).
Prior to Angrim's assignment to the Ithilien Rangers, he was ordered by Denethor to provide information on Faramir's conduct of his captaincy, a task he carried out until at least the battle with the Southrons in March 3019 (see Intent). The night before the battle, Angrim confessed to Faramir that he had been Denethor's spy in the company. Faramir was already aware of the fact, and had been for some time prior to the breaking of the bridge at Osgiliath in June 3018 (see Suspicion).
Angrim had a reputation as a "man of letters" amongst the company.
Angrim's name was chosen to give a clue to his treacherous nature: in "Of Beren and Luthien" (The Silmarillion), Angrim is the father of Gorlim the Unhappy, who betrays Barahir to Sauron.
It is not clear whether Angrim ever revealed Hethlin's presence in the Ithilien company to Denethor. It is also not clear to what extent Húrin, Denethor's spy-master (and brother-in-law) was aware of Angrim's special responsibilities concerning his nephew-by-marriage, Faramir.