Donald George Morrison, Robert Cameron Mitchell, and John Naber Paden's book Black Africa: A Comparative Handbook (2nd edition) (Macmillan Reference Books, 1989) contains, among others, the list of ethnic groups and the background of heads of government (age in 1982, ethnicity, education, former occupation) for each country in sub-Saharan Africa.
The information on the ethnicity of heads of government is used by Londregan et al. (1995) "Ethnicity and Leadership Succession in Africa", International Studies Quarterly, 39: 1-25. But it contains a few mistakes:
Page 446: Mengistu Haile-Mariam's ethnicity is Wollamo, not Amhara. (See Clapham 1989, p. 12; Erlich 1976, p. 636; Samatar 1984, p. 718). Haile (1986) cites, "His culture and upbringing is Amhara, although his parentage is of diverse minority sources" (page 474). Londregan et al. (1995) say, "Mengistu Haile Mariam in Ethiopia was said to be the son of a woman descended from Galla (Oromo) slaves..." But I cannot confirm this claim from other sources.
Page 509: Daniel Arap Moi's ethnicity is Tugen, not Kipsigis. (I learn this from a couple of Kenyan specialists.)
Page 704: Kenneth David Kaunda's ethnicity is not clear-cut. Posner (2005) points out that Kaunda was identified as a Bemba until 1971, after which he was associated with Nyanja (see footnote 12 in chapter 4).
See page 6 of Londregan et al. (1995) for further information on the ethnicity of Hissan Habre of Chad, Thomas Sankara of Burkina Faso, and Macias Nguema of Equatorial Guinea.