There was the already-Christian Ethiopian Empire (1270-1974). There was the Wadai Empire in Chad (before the French took over in 1918, anyway). There was the Kingdom of Kongo, which went on for 500 years or so before Portugal suppressed the monarchy and absorbed it into Angola.
There was the Empire of Benin, one of the longest-lasting, most developed of any African empire… before the British came along in 1897. The list goes on.