The value of a 1942 Jefferson nickel depends significantly on which of the two distinct versions you have. The U.S. Mint produced both a standard copper-nickel version and a special silver alloy version during that year due to the strategic importance of nickel for the war effort.
The first type is the standard composition nickel, made of 75% copper and 25% nickel. These were struck at the Philadelphia mint (no mint mark) and the Denver mint (a small "D" mint mark to the right of Monticello). In circulated condition, these coins are extremely common and are typically worth only their face value of five cents. For collectors, an uncirculated example that shows no wear can be worth more, with values ranging from a few dollars to upwards of $25 for a coin in pristine MS65 condition.
The second, more valuable type is the "war nickel," which was struck later in 1942. To conserve nickel for military use, the composition was changed to an alloy of 56% copper, 35% silver, and 9% manganese. The key identifier for this silver version is a large mint mark (P, D, or S) located directly above the dome of Monticello on the reverse of the coin. Because of their silver content, these nickels have an intrinsic metal value that is well above face value, even in heavily worn condition. Uncirculated 1942 silver nickels are also sought by collectors and can be worth from approximately $8 to $30 or more, depending on their specific grade and condition.
Therefore, the first step in determining the value is to check the reverse side of the coin for a large mint mark above Monticello. Its presence indicates you have the silver version, which is inherently more valuable. The final value for either type is ultimately determined by its physical condition or grade.
