Mainardo I of Tirolo-Gorizia (formerly Mainardo III of Gorizia) was Count of Gorizia and Istria (1220-1258) as Mainardo III of Gorizia and then also Count of Tirolo (1253-1258) as Mainardo I of Tirolo-Gorizia. He was the son of Enghelberto III count of Gorizia (died in 1220) and of Matilde di Pisino (died in 1245), probably daughter of Bertoldo III of Andechs.
Mainardo sided with the Staufen and Frederick II of Swabia made it in 1246 Lieutenant of Styria and, four years later, of Austria, soon stripped of his powers from Ottocaro II of Bohemia. He then tried to take Carinthia, but failed, despite having his father-in-law Alberto III of Tirolo at his side, losing the battle of Greifenburg against von Spanheim. The subsequent peace provided, in addition to the payment of heavy credits and the cession of some cities, the delivery of the sons of Mainardo (Mainardo and Alberto) as hostages. They remained in Werfen until 1258 or 1259.
On the death of Albert III of Tirolo in 1253 his daughter Adelaide, wife of Mainardo, inherited part of his titles and possessions. In Mainardo, therefore, touched the southern Tyrol, with the then capital Merano. He died in Castel Tirolo, where he was buried, in 1258.
After his death the two counties were again divided among their children. In Mainardo II it was the county of Tirolo and the title of count of Tirolo-Gorizia, to Alberto went the county of Gorizia with the title of Count of Gorizia-Tirolo,