Jenckes was first elected to the U.S. House in 1863, defeating Democratic opponent Charles J. Bradley by seventeen percentage points.

[5] Contrary to some narratives that suggest this was an effort to bolster Reconstruction policies during the presidency of Ulysses S. Grant and safeguard the constitutional rights of blacks, it was primarily for the purpose of professionalizing legal practice.

"[5] When he introduced legislation pushing it, fierce opposition was met by Radical Republican colleague John A. Logan of Illinois,[8] who was concerned with the plight of Southern blacks and called the bill "probably unconstitutional.

[10] Having prosecuted fraud implicated in the Credit Mobilier scandal, Jenckes' tenure was also marked by his revisions of bankruptcy law and involvement in cases related to Goodyear rubber, the Corliss steam engine, in addition to patents pertaining to refrigeration and ventilation.

[11] Jenckes died in Cumberland, Rhode Island on November 4, 1875, and is interred at Swan Point Cemetery in Providence.