The activity of Transposable Elements (TEs) represents an endogenous source of mutagenesis capable of generating genetic diversity within mammalian genomes. While all ongoing TE activity in the human genome is derived from Non-Long Terminal Repeat (non-LTR) retrotransposons such as Long Interspersed Element-1 (LINE-1 or L1) and short interspersed element (SINE) retrotransposons, the mouse genome is subject to both non-LTR and LTR retrotransposon mutagenesis. We previously identified de novo L1 insertions arising in pedigrees of C57BL6/J mice and traced their developmental origins to pluripotent embryonic cells and early primordial germ cells (PGCs). Here, we identify both de novo LTR retrotransposon insertions and de novo L1 insertions in an inter-strain C57BL6/J x A/J mouse pedigree. We employ a PCR genotyping strategy to trace the developmental origins of de novo retrotransposon insertions and use long-read sequencing to fully resolve their internal sequences, structural hallmarks, and somatic cell DNA methylation profiles. This study expands our understanding of the developmental origins of TE mutagenesis to encompass LTR retrotransposon activity.