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In this paper, we consider a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system operating over unknown frequency-selective fading channels. Joint maximum-likelihood (ML) estimation of carrier frequency offset (CFO) and MIMO channel coefficients is prohibitively complex for practical purposes. To overcome this difficulty, we resort to the expectation-maximization (EM) algorithm and propose a novel scheme for joint ML estimation of CFO and channel parameters which is suitable for practical applications. Moreover, the Cramer-Rao bounds (CRB) for both CFO and channel estimators are developed. Computer simulations show that the proposed algorithm achieves almost ideal performance compared with the CRB for both channel and frequency offset estimations.