We present a multichannel quantum-defect theory for magnetic Feshbach resonances in the interaction of two heteronuclear group I atoms. The theory provides a unified and a uniform description of resonances in all partial waves, and enables the characterization of large number of resonances in terms of very few parameters. For the sample system of ^6Li^40K, we present descriptions of all resonances in aa, ab, and ba channels, in partial waves s (l=0) through h (l=5), and in a magnetic field of 0

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... through 1000 Gauss. All resonances, including those in nonzero partial waves, are fully characterized using the newly developed parametrization of Gao [Phy. Rev. A 84, 022706 (2011)].