  We report measurements of the isotope shifts of the (3s3p)$^3$P$_{0,1,2}$ -
(3s4s)$^3$S$_1$ Mg I transitions for the stable isotopes $^{24}$Mg (I=0),
$^{25}$Mg (I=5/2) and $^{26}$Mg (I=0). Furthermore the $^{25}$Mg $^3$S$_1$
hyperfine coefficient A($^3$S$_1$) = (-321.6 $\pm$ 1.5) MHz is extracted and
found to be in excellent agreement with state-of-the-art theoretical
predictions giving A($^3$S$_1$) = -325 MHz and B($^3$S$_1$) $\simeq 10^{-5}$
MHz. Compared to previous measurements, the data presented in this work is
improved up to a factor of ten.
