He was a descendant of Húrin of Emyn Arnen, an earlier Steward of Gondor and the grandfather of Mardil Voronwë, the first Ruling Steward of Gondor.
Pelendur helped Gondor through a constitutional crisis following the premature death of King Ondoher and his heirs in battle. He effectively reigned the kingdom for one year while observing the claims of the two claimants, Prince Arvedui of Arthedain (the one with the stronger claim) and General Eärnil who was victorious against the Wainriders (Easterlings).
Pelendur resisted the rightful claim of Arvedui because he favoured a king from Gondor despite Arvedui's statement that the death of Isildur was a mere accident and that it was never meant to be that Meneldil would become king of Gondor.
Based on the afore-mentioned data the Council of Gondor under the influence of Pelendur threw out Arvedui's claim and chose Eärnil as King, defying the Númenorean law of succession and severely weakening the position of both Gondor and Arthedain who would have fared well under a common ruler.