His early education included instruction in the art of perspective, and he may have studied quadratura (scenic perspective or design) with Ferdinando Galli Bibiena. He probably began painting in Piacenza, but his early activity remains entirely conjectural. Pannini settled in Rome in 1711 and shortly thereafter entered the studio of Benedetto Luti.

In 1718–19 Pannini was admitted into the Academy of St. Luke. His reception piece, “Alexander Visiting the Tomb of Achilles” (1719), is typical of his earlier easel paintings, having small figures dwarfed by an elaborate architectural construction derived from Bolognese theatrical scenography. Many of his canvases prior to 1730 feature explicit historical or religious subjects. His frescoes at the Villa Patrizi (1718–25, later destroyed) established Pannini's fame in this field. Later decorations include those at the Palazzo Alberoni (c. 1725; now Senato Palazzo), displaying his talent as a quadraturist, and at Santa Croce in Gerusalemme (c. 1725–28).

Toward 1730 Pannini began to specialize in the depiction of Roman topography. To satisfy tourists' demands for his paintings, Pannini frequently repeated subjects yet always retained his spontaneity by varying composition and details. Pannini's oeuvre included interiors of Roman buildings, old and new; most famous are the many versions depicting the Pantheon and St. Peter's. He was admitted into the French Academy in 1732 and subsequently became its professor of perspective. His greatest pupil was Hubert Robert. In 1754 Pannini became principal of the Academy of St. Luke. He painted little after 1760.