Background: Bone and soft tissue sarcomas are among the most common extra-cranial solid tumors of childhood, yet advances in liquid biopsies lag behind what has been achieved across adult cancers. Solid tumors that lack recurrent genetic mutations or translocations, such as bone and soft tissue sarcomas, are more difficult to detect in circulation using established assays, such as ddPCR. We have developed a pipeline to analyze cell-free DNA (cfDNA) using two molecular profiling techniques that we hypothesized would improve detection of circulating tumor DNA at diagnosis for patients with bone and soft tissue sarcomas. Methods: cfDNA was isolated from plasma of patients with osteosarcoma (OS) (n = 36) and fusion-negative rhabdomyosarcoma (FN-RMS) (n = 7) at multiple timepoints. Methods for detecting tumor DNA in cfDNA included ultra-low coverage whole genome sequencing (ULC-WGS, 0.5x) and ultra-deep targeted sequencing (UDP, ~1000x) using a panel of 11 and 22 tumor-associated genes for OS and FN-RMS, respectively. ULC-WGS data underwent ichorCNA analysis to define percent tumor content of each sample. UDP analysis identified somatic variants through variant callers, Mutect2, Lancet, strelka, and DELLY2, with ClinVar to annotate and define tumor-specific variants in each sample. Combining ULC-WGS and UDP data, an integrated tumor score was developed and assessed in diagnostic samples. Results: 136 OS and 30 FN-RMS cfDNA samples were collected during the patients’ disease courses. Circulating tumor material was detected in 18/23 (78%) OS and7/7 (100%) FN-RMS samples at diagnosis. The most frequent copy number aberrations were seen in the region of chromosome 8q containing the cMYC gene. cMYC amplification was found in 11/23 (48%) of diagnostic OS samples and 2/7 (29%) of diagnostic FN-RMS samples. UDP analysis identified variants in 17/23 (73%) and 7/7 (100%) of diagnostic OS and FN-RMS samples respectively. The most commonly detected variants included TP53, BRCA2, MET, and BRAF. Of the 23 diagnostic OS samples, four had TP53 translocations. The integrated tumor score detected the presence of tumor associated material within diagnostic samples with a sensitivity of 25/30 (83%), compared to the sensitivity of ULC-WGS or UDP alone, which was 15/30 (50%) and 24/30 (80%) respectively. Conclusions: A multi-assay liquid biopsy has promise to improve disease detection and monitoring for patients with solid tumors that lack recurrent driver mutations. We plan to integrate additional molecular profiling technologies to improve the sensitivity and specificity of our cfDNA assays and aim to adapt our pipeline across additional cancers.