Frontenac Provincial Park is located on the Frontenac Axis, a southern extension of the Canadian Shield, linking Algonquin Park to the Adirondack Mountains. To better understand the lichen biota of this interesting ecosystem, an inventory was conducted during several field visits from 2016–2019. During these surveys, 280 species of lichens and allied fungi in 115 genera were discovered. Presented is the first published record of Lempholemma cladodes (Tuck.) Zahlbr. in Canada, as well as the first published discoveries of Cladonia petrophila R. C. Harris, Coccocarpia palmicola (Spreng.) Arv. & D. J. Galloway, and Leprocaulon adhaerens (K. Knudsen, Elix & Lendemer) Lendemer & B. P. Hodk. in Ontario. Sixteen species are provincially ranked as critically imperilled (S1, S1S2 or S1S3), sixteen species as imperilled (S2 or S2S3), and nineteen species as vulnerable (S3 or S3S4). Unranked species reported for the second time in Canada are Dermatocarpon muhlenbergii (Ach.) Müll. Arg. and Cladonia atlantica A. Evans. An undescribed species of Lempholemma growing on flooded deciduous tree bases in vernal pools was also discovered. A discussion of Lobaria pulmonaria (L.) Hoffm., a pollution and habitat disturbance sensitive species, is presented. The records of L. pulmonaria discovered in Frontenac quite possibly represent the most southern observations in the province and are of conservation concern. It is recommended that a lichen awareness and education program be created for the park staff and visitors to highlight these rare and sensitive lichens and habitats within the park in the hope that lichens are protected through appropriate management and planning.