[3][4] Banksia plagiocarpa was first collected on 20 December 1867 and 21 February 1868 by John Dallachy (a collector for Victorian Government Botanist Ferdinand von Mueller) from Bishops Peak in the Cardwell Ranges, the specimens lodged at Kew and Melbourne Herbariums.

The species name is derived from the Ancient Greek words plagios "sloping" or "oblique", and carpos "fruit", and refers to the upturned wedge-shaped follicles.

[5] The current taxonomic arrangement of the genus Banksia is based on botanist Alex George's 1999 monograph for the Flora of Australia book series.

[3] In a morphological cladistic analysis published in 1994, Kevin Thiele placed it in the newly described subseries Acclives, within the series Salicinae.

Their analyses suggest a phylogeny that differs greatly from George's taxonomic arrangement, and had some similarities to Thiele and Ladiges', thus Banksia plagiocarpa grouped in a clade with B. aquilonia, B. oblongifolia and B. robur.

[7][8][9] A 2013 molecular study by Marcel Cardillo and colleagues using chloroplast DNA and combining it with earlier results reaffirmed B. plagiocarpa as an offshoot of a lineage that gave rise to B. robur, B. oblongifolia and B.

[10] Early in 2007, Mast and Thiele rearranged the genus Banksia by merging Dryandra into it, and published B. subg.

[2] Banksia plagiocarpa adapts readily to cultivation, and grows well with good drainage and a sunny aspect.